I’ve been what seems like halfway through this exact project for 6 years now. I did it once before (many years ago) using a 16TS engine and a 4sp 15TS box as you’re proposing and it involved a lot of fab and a lot of problem solving, so I feel I’ve made all the mistakes already.
The latest version of the project is doing as everyone has suggested and I have used a donor 17 for everything I need, and for everything I already had, now I have spares. With a complete 15/17 donor car, it is quite a straight forward process, but as David said, you’ll likely need to source manifolds and side draft carbs or go down the injection route, which is what I’ve done, but this opens up more cans of worms and the project snowballs like mine.
Its just my opinion, but honestly, if I was starting this project again now, I’d source the bigger brakes (I’ve come across the hubs, rear caliper mount brackets, calipers, etc, in separate ads online), get the bigger swaybars from signature, find a UN1 box, get some driveshafts made and drop in an electric motor for an EV conversion, but I’m also a little disillusioned with the project just at the moment.
Hang in there! You are so close now!